Recently, the demand for portable electronic products such as notebooks, video cameras, cellular phones or the like is rapidly increasing, and energy storage batteries, robots, satellites or the like are developed in earnest. Accordingly, a high-performance secondary battery allowing repeated charging/discharging is actively studied.
Secondary batteries commercially used at the present include nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, lithium secondary batteries or the like, among which the lithium secondary batteries are in the limelight due to their very low self-discharge ratio, high energy density and free charging/discharging since a memory effect does not substantially occur in comparison to nickel-based secondary batteries.
Recently, due to exhaustion of carbon energy and increased interest in the environment, hybrid electric vehicles and electric vehicles are becoming the focus of public attention all over the world, including US, Europe, Japan and Korea. For such vehicles, the most important component is a vehicle battery which gives a driving force to a vehicle motor. Since the hybrid electric vehicle or the electric vehicle obtains a driving force by means of charging/discharging of the battery, they ensure better mileage in comparison to a vehicle using only an engine and discharges little to no pollution, which is one of the growing reasons why people choose to use such vehicles.
Meanwhile, one of the most important social issues in relation to batteries is safety. The use of batteries is not only increasing in small-sized devices such as notebooks and cellular phones but also in middle-sized or large-sized devices such as vehicles and power storage systems. Also, since the explosion or fire of a battery may damage a device to which the battery is mounted and can lead to additional conflagration, the safety of the battery is of major concern.
In particular, middle-sized or large-sized batteries used for hybrid electric vehicles or electric vehicles demand even stricter safety requirements. Such middle-sized or large-sized batteries generate high voltage, which in itself cause a lot of heat. Moreover, in cases of a battery for a vehicle, if a battery pack is excessively operated at high temperature during the hot summer time or great impact is applied to the battery due to an accident such as vehicle collision, causing an internal short circuit or a protection function to malfunction, a fire may occur at the battery for a vehicle.
If a fire occurs at the battery for a vehicle as described above, the fire is not limited to damaging just the battery but can spread to the vehicle, which may obviously cause property damage, but also lead to risking the safety of the person in the vehicle. Moreover, on the road, a vehicle fire may spread to other vehicles and cause traffic congestion, which may result in even more serious damage.